1. Field of the Invention
The present invention relates generally to the real-time location of Mobile Units by correlating an on-demand radio signal received by multiple antennae. The Mobile Units are equipped with a transceiver that is activated by an encoded broadcast and responds with a burst transmission that is received by several well-surveyed antennae at slightly different times due to the variable distance of the Mobile Units to the Fixed Receiver antennae. The location of a Mobile Unit is determined by resolving the time delay differences from the antennae.
2. Related Art
There are other methods of locating the source of a radio broadcast in real-time, however, each has performance or implementation shortcomings.
The use of fixed directional antennae to receive an omni-directional broadcast from the source to be located allows a line-of-sight correlation from two or more receiver locations to determine the location of the broadcast point. The number of receivers required to determine the broadcast source is dependent on the volume of the space under regard. The search volume is divided into segments radiating from each fixed receiver and an antenna element is dedicated to each segment. The more precision desired in the location determination requires more segments and depending on the configuration of the search volume more antennae. The infrastructure necessary to achieve the location precision achievable with the present invention is much more complex for this approach than the infrastructure of the present invention.
Other implementations of real-time electronic locating use the varying time delay to multiple receivers of a periodic transmission from the mobile unit to be located to correlate the position of the source. This approach uses a mobile unit-generated broadcast with a synchronized and precise time tag to allow the time variability of receipt at multiple, fixed, remote receiving antennae to be correlated to a single accurate location. The use of mobile unit-generated signals necessitates a continuous tracking system and does not provide for privacy when the mobile unit to be located is an individual. The period of the mobile unit broadcast is fixed for each mobile unit and a broadcast spectrum allocation made to accommodate each mobile unit. The allocation of broadcast spectrum introduces an inherent limitation to the quantity of deployed mobile units. This technology has been successfully deployed as an inventory management tool where the multiple sources to be tracked are relatively slow moving allowing an extended period between broadcasts for each source. In addition, the inventory management broadcast equipment is readily re-used and compatible with the expense of many synchronized, precision clocks.
There are also real-time locating systems that use the Global Positioning System as the source of location information and wireless communications infrastructure for reporting the mobile unit position to a master control center. The limitations of the GPS low signal power in penetrating structures and dense foliage create significant system coverage outages and reduce the utility of the method. The mobile equipment requires both a GPS receiver and a wireless communications device and is packaged in a larger physical configuration than the present invention. The absence of a universal standard for wireless communications and the lack of ubiquitous coverage limit the operational utility of this approach.
The present invention is an on-demand real-time locating system for people or products using radio frequency broadcasts. The system uses an unlicensed family use radio frequency in the 40014 450 MHz band in confined areas of concern by placing Fixed Receiver antennae within and on the periphery of the area. The present invention supports the real-time locating of multiple low signal power broadcast sources within the range of the fixed receivers by using digital frequency re-use techniques, unique digital identification codes, and parallel processing of a time ambiguous mathematical determination of the location of each source.
A master broadcast center of the present invention transmits a high power radio frequency signal throughout the volume of concern to be received by all mobile receivers attached to the subjects to be located. The master signal structure is a digital construct containing a reference time tag and a string of activation codes for the mobile receivers of interest in each packet. The packets are broadcast in a continuous series activating a different set of codes with each packet. The high power signal can penetrate the local area structures and activate the mobile transponders anywhere within the volume of concern.
The mobile transponder of the present invention responds to the activation signal by generating a low power broadcast on a separate, but also 400-450 MHz band, frequency. The low power signal minimizes the size, complexity, battery power needs, and cost of the Mobile Unit and makes it less intrusive for personal use. The low power signal, however, requires more than the minimum number of Fixed Receiver antennae because the signal is more easily interfered with by local structures. The present invention uses a Fixed-Receiver-geometry-sensitive determination to choose the best available combination of Fixed Receivers to resolve the location of the mobile transponder.
The time ambiguous mathematical determination of the mobile transponder location at the time of the low power broadcast is accomplished by selecting the four best Fixed Receivers to provide the most diverse geometry to provide the minimum location error. The mathematics simultaneously solves four equations to permit the resolution of the mobile transponder location in three physical dimensions and without a synchronized clocking system for the mobile transponder and Master Station. By eliminating the need for an accurate and synchronized Mobile Unit clock the Mobile Unit complexity, size, and costs are reduced.
Each Mobile Unit is provided with a unique digital identification via radio frequency identification technologies. The radio frequency identification is used to pinpoint the location of a Mobile Unit by using a portable radio frequency interrogator, when necessary. Further, the radio frequency identification can also be used to provide proof of identification and authorization for requests to locate other Mobile Units when used for tracking people or personal property. The central processing center is provided with an access database to correlate individuals of groups with their associates. A member of a group is authorized to request the real-time location of any or all of the members of their group.
Further, when applied to locating people the Mobile Unit includes an integrity band secured around the wearer""s wrist. Any broach of the integrity of the band will trigger a broadcast from the Mobile Unit containing a message for the central processing center. The message alerts the system of the security breech and identifies the location of the incident. Each Mobile Unit retains the time-tag from the latest master broadcast center broadcast regardless of the whether its activation code was included in the message. Depending on the purpose of the system, the features associated with the real-time locating and identification verification capabilities are disabled by the incident.